Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

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The satellites were Cosmos 2251, a Russian communication relay satellite that's been defunct for a decade, and an Iridium satellite, one of a fleet of communication satellites launched by Motorola in the late 90s and early 2000s.

The speeds involved are somewhat higher than your standard traffic collision. Satellites at this height above the Earth -- about 800 kilometers -- have an orbital speed of about 8 km/sec. However, these two satellites were on similar orbits, tilted with respect to each other by about 12 degrees. If I have accurate numbers, the physics of the situation means the collision speed was about 1.5 km/sec or so, which is pretty dang fast. Because of the way orbits work, this is about the same as the actual collision speed.

There have been collisions in space before, but never from such large satellites -- the Iridium bird was about 700 kg, and the Cosmos was about the same -- and never resulting in a total wipeout like this. Again, if I have my numbers about right, the explosion resulting from the energy of impact would have been about the same as detonating a ton 5 tons of TNT.

That's a lot. It's easily enough to totally destroy both satellites, and in fact the U.S. Space Surveillance Network has detected a substantial amount of debris, at least 600 pieces.

At the moment, that debris is expanding in a cloud, and is still too high to threaten the space station which orbits at less than half the height where the satellites collided... but eventually the debris will pass through the altitude of the ISS. It's not clear yet how much danger the station is in. Satellites in similar orbits as the two that hit are in the most immediate danger, but again it's unclear what will happen.